E. coli F' DNA carrying the gene for guanine-zanthine phosphoribosyltransferase (gxu) will be purified from F' carrying cells. Lambda phage carrying the gxu gene will be obtained by inducing lambda prophage that have integrated next to the chromosomal gxu gene. F' and lambda DNA will be added to human cells lacking a functional gxu gene (obtained from Lesch-Nyhan patients) in tissue culture. Cells with a functioning gxu ion gene will be identified by growing the cells in tritiated guanine and examining for tritium incorporation by autoradiography. The genes for gxu and hypoxanthine phosphoribosyltransferase will be studied in E. coli by examining the enzymes from wild type and mutant cells.